I just picked up some .375 RUM brass to size and use in my .338-300 (Edge). Before and after I size the brass in my press, the shoulder is around .020 to .024 shorter than my fired brass. I used a Hornady/Stoney Point cartridge headspace bushing gauge to measure. This is the first time I have checked the headspace of a sized and unfired brass. The reason I am being careful now is I recently switched from Nosler 300 Ultra brass to the Rem 300 brass. On the 3rd firing of the remington I had a cartridge base seperate, and others getting thin in the same spot(about 1/2" up from base). My first thought was that I had undersized the brass(since I am using a .338 ultra die, it is possible if I wasn't paying attention) I bought the gauge tonight to check the size of the rest of the unfired brass from the group where I had the brass seperate. They were either the same size or less than the fired brass by .003" Now I am wondering if the 300 ultra remington brass was similarly short for the chamber on initial forming as I am now seeing with the 375 brass. It would explain the case seperation, which should only happen with excessive headspace. Another note, the fired brass is .001 shorter than the Go Gauge I used to set the headspace on the rifle(Savage 110, PacNor prefit barrel) I also checked headspace on the rifle and it is fine. As well as the fired Nosler brass is the same dimensions as the fired Remington, so headspace hasn't changed. Long story short. It looks like I'm going to be fireforming for my 338 edge. Does anyone else have this problem? Or any ideas what I could be missing? thanks in advance guys.